The invention relates to the evaluation of ionic current signals for judging combustion operations.
In combustions, an ionization of the participating gases takes place because of chemical and physical processes. When a voltage is applied to two electrodes, which are insulated from each other and which project into the gas, a current can be measured which is carried by the ions in the gas chamber. This is referred to in the following as an ion flow.
For combustion processes in internal combustion engines, for example, in spark-ignition engines, it has long been attempted to utilize the ionic current for various engine control and diagnostic functions, for example, for knock detection, misfire detection, estimating combustion pressure and/or the position of pressure maximum, determination of mixture composition and detection of the lean running limit.
The spark plug is used conventionally as a measuring probe. After applying a voltage across the center electrode and ground, the ion flow can be measured after decay of the ignition spark. Apparatus possibilities for detecting ionic current signals in this context are disclosed, for example, in U.S. Pat. No. 5,220,821. The ionic current signal can be detected in the high voltage loop as well as in the low voltage loop of the ignition system.
The invention relates to method as well as apparatus aspects in combination with an extraction of features from the ionic current signal to evaluate the combustion.
The detection of combustion misfires is of primary concern.
A conventional method for detecting combustion misfires is the successive integration of the ionic current signal over a pregiven measuring window region. The integration value, which is obtained at the measuring window end, is applied as a feature for classification between combustions and misfires.
If disturbance components are superposed on the measured ionic current signal, then the disturbance distance deteriorates with increasing length of the integration window. To still make possible a classification at operating points with weak ionic current signals, the disturbances can, to a certain extent, be excluded by limiting the length of the integration window.
The possibility of the above-mentioned limiting is, however, limited by another effect, namely, the region in which an ionic current signal can be measured, can shift considerably in dependence upon the operating parameters (for example, engine speed, air/fuel ratio, et cetera). Long integration windows can be so positioned that they reliably include also the shifted regions. The mentioned limiting of the integration window leads, however, to the problem that the shortened windows, under circumstances, can no longer reliably include the above-mentioned regions or that the position of the shortened integration windows relative to the reference angle positions of the crankshaft and/or camshaft have to be adapted with much complexity to the conditions of individual engine types.
In this context, it is the object of the invention to provide an arrangement and a method for evaluating the ionic current signal with further increased reliability of the evaluation of the quality of combustion processes without increased adaptation complexity.
A significant feature of the invention is the substitution of a long integration region with a shorter integration region, which is so shifted in its position, that it covers the long integration region in combination with the displacement.
During the displacement, integration takes place repeatedly for short times and the value of the integrator is reset to a starting value with each new integration. The maximum value of the different results of the short-time integration obtained in this manner is used for the evaluation of the combustion quality, for example, for the detection of combustion misfires.
The method of the invention is characterized, on the one hand, in that the actual integration, in each case, can be limited to a short time duration. Because of the shortness, the noise component can be summed only to a limited extent. In this way, the method of the invention is robust compared to noise components, that is, insensitive.
On the other hand, the sliding displacement of the short time integration region over the entire monitoring time span of interest makes possible an adaption of the position of the integration region to the context conditions of an individual internal combustion engine type with advantageously low complexity.
Use signal components which project out of the noise are detected in one of the short-time integrations and can be identified by a subsequent maximum value selection. For this reason, the invention provides, in addition, the advantage of a high reliability in the evaluation of combustion quality, especially in the detection of combustion misfires.
This method supplies a significant improvement of the signal-to-noise ratio especially when signal noise (base noise) is present.
Investigations have, however, also shown that disturbance components having large amplitudes are present in the ionic current signal in addition to base noise and these components greatly disturb the method.
The large amplitudes of these disturbance components make it difficult to reliably distinguish between regular combustions and combustion misfires because these disturbances with successive integration provide values similar to combustions.
Combustion misfires which occur in parallel are then possibly no longer reliably detected. This can be observed especially at idle of the engine.
Because of statutory requirements, misfires must also be reliably detected in the idle region. In the context of a further embodiment of the invention, this is achieved via a suppression of short disturbance pulses in the processing of the ionic current signals.
Variations of the invention can be used alternatively or supplementary to the misfire detection also for the extraction of additional features for the recognition of running limits in leaning mixtures. The running limit is characterized by an increase of delayed combustions.